It is common to provide an exhaust extraction system for extracting noxious fumes from one or more welding sites. The exhaust fumes are conveyed from each work site through a primary duct, the primary ducts are connected to a secondary duct and an extraction fan extracts the noxious fumes from the secondary duct and expels the fumes from the building in which the work sites are located. While it is common to provide a damper in each of the primary ducts, the damper control mechanism can be a manually operable mechanism which is designed to be operated by the welder. If the damper is not closed when the welding machine is not in operation, air will be withdrawn from the work site through the duct system so as to be expelled from the building. The air which is expelled must be replaced by drawing any air from the exterior of the building. This fresh air which is drawn into the building must then either be heated or cooled to maintain the required working temperature within the building, and as a result a considerable amount of energy may be used to heat or cool the replacement air which is required to replace uncontaminated air which is expelled unnecessarily through the extraction system while the manually controlled damper mechanism so designed to reduce this problem, in practice, welders usually open the damper when they begin their working day and leave the damper open until the end of their working day. In many instances, the hand welding machine is actually operating for less than 50 per cent of a working shift.